. >> reporter: most memorably, lloyd bentsen's takedown of dan quayle in 1988. >> jack kennedy was a friend of mine. senator, you're no jack kennedy. >> reporter: but beware that deer in the head lights moment when a candidate forgets he's expected to be human. >> governor, if kitty dukakis were raped and murdered, would you favor an irrevocable death penalty for the killer? >> no, i don't, bernard. >> i feel sorry for these candidates because there's a bunch of people sitting around a room right now telling them to do ten impossible things and then at the end saying be yourself. >> reporter: and if you lose the first round you can recover at the next match with a well executed zinger. >> i am not going to exploit for political purposes my opponent's youth and inexperience. >> reporter: even a strong debate performance by john kerry wasn't enough to defeat george w. bush, but debates matter because it's the first chance to size up the candidates in unscripted moments one-on-one. brian? >> andrea mitchell, who by the way will be our fact checker during our live coverage of the debate